#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
string s,t;
ll sum=0;
char q[26]={'b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','a'};
char v[10]={'1','2','3','4','5','6','7','8','9','0'};
int main()
{
	cin >> s;
	cin >> t;
	for(int i=0;i<s.size();i++)
	{
		if((s[i] >= 'a' && s[i] <= 'z' && t[i] >= '0' && t[i] <= '9') || (s[i] >= '0' && s[i] <= '9' && t[i] >= 'a' && t[i] <= 'z'))
		{
			cout << "IMPOSSIBLE";	
			exit(0);		
		}
	}
	while(1)
	{
		if(s==t)
			break;
		for(int i=0;i<s.size();i++)
		{
			char f=s[i];
			if(s[i] >='a' && s[i] <= 'z')
				s[i]=q[f-'a'];			
			else if(s[i] >='0' && s[i] <= '9')
				s[i]=v[f-'0'];
		}
		sum++;
 	}
	cout << sum;
	exit(0);
}